This invention relates to gas turbine fuel delivery systems, and relates more particularly to an improved method and apparatus for pneumatically purging the fuel manifold during engine shutdown.
Increasingly stringent governmental environmental pollution standards have required the reduction and elimination of emission of unburned hydrocarbons from gas turbine engines. This is particularly troublesome during shutdown operation of such an engine when a substantial volume of fuel remains in the fuel manifold subsequent to shutoff of pressurized fuel flow, the remaining fuel in the manifold is subject to relatively slow leakage into the combustion chamber. Continued air flow through the combustion chamber carries the unburned fuel into the atmosphere. The most successful method, to date, of eliminating this emission of unburned hydrocarbons into the atmosphere has included the drainage of fuel from the manifold, its collection and reintroduction into the engine during the next operating period. Such a method is only partially successful since a substantial portion of the fuel in the manifold may still drain into the combustion chamber rather than into the drainage collection container dependent upon the geometry of the fuel manifold. Further, this arrangement adds excessive weight and takes a substantial portion of valuable space in the engine.
It has also been attempted to provide a purging system for the fuel manifold which includes utilization of a pneumatic accumulator charged with pressurized gas by the compressor of the turbine engine, which pressurized gas is delivered to the fuel manifold upon engine shutdown in order to blow the fuel out of the manifold and into the combustion chamber. This arrangement for gas turbine engines, however, still does not reduce unburned hydrocarbon emissions during shutdown since the raw fuel is simply blown into and carried out of the combustion chamber into the atmosphere. Other arrangements utilized in fuel delivery systems not associated with gas turbine engines contemplate a separate gas compressor which is continually operative to maintain a substantially constant gas pressure source for purging a fuel line coincident with shutdown of the device. Such arrangements are not applicable to the gas turbine field because of the substantial weight, space and additional expense involved in providing a separate, continuously driven compressor. Indicative prior art systems of the type referred to can be found in U.S. Pat. Nos. 2,056,568 of Davis; 2,712,218 of Ritter; 2,818,110 of Rulsch; 2,837,148 of Jay; 3,344,602 of Davies et al; and 3,498,056 of Avery.